Method and system for adaptive peak to average power ratio reduction in orthogonal frequency division multiplexing communication networks

ABSTRACT

A method and system adaptively reduce a peak-to-average power ratio in a communication system. Energy is clipped from at least one peak of a modulated signal. The modulated signal includes a plurality of sub-carriers. At least one data sub-carrier is adaptively selected for peak-to-average power ratio reduction use based on known scheduling information. The clipped energy is distributed among at least one data sub-carrier.

PRIORITY CLAIM

This application is a continuation of U.S. patent application Ser. No.12/245,047 entitled “METHOD AND SYSTEM FOR ADAPTIVE PEAK TO AVERAGEPOWER RATIO REDUCTION IN ORTHOGONAL FREQUENCY DIVISION MULTIPLEXINGCOMMUNICATION NETWORKS” to Guo et al., filed Oct. 3, 2008, which claimspriority to U.S. Provisional Application Ser. No. 60/977,403, filed Oct.4, 2007, entitled ADAPTIVE PAPER REDUCTION, all of which areincorporated herein by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

n/a

FIELD OF THE INVENTION

The present invention relates generally to a method and system for powercontrol in communication systems and more specifically to a method andsystem for reducing the peak-to-average power ratio (“P APR”) byadaptively distributing excess energy among reserved and activesub-carriers in orthogonal frequency-division multiplexing (“OFDM”)communication systems.

BACKGROUND

The use of Orthogonal Frequency Division Multiplexing (“OFDM”)technology is ever increasing within wireless applications such ascellular and Personal Communication Systems (“PCS”) due to itsreliability and high spectral efficiency. OFDM has a high tolerance tomultipath signals and is spectrally efficient which makes it a goodchoice for wireless communication systems. OFDM has gained considerableinterest in diverse digital communication applications due to itsfavorable properties like high spectral efficiency, robustness tochannel fading, immunity to impulse interference, uniform averagespectral density, and capability of handling very strong echoes. OFDMtechnology is now used in many new broadband communication schemes andmany other wireless communication systems.

More specifically, OFDM is a special form of multicarrier modulationthat uses Digital Signal Processor (“DSP”) algorithms such as InverseFast Fourier Transform (“IFFT”) to generate waveforms that are mutuallyorthogonal and Fast Fourier Transform (“FFT”) for demodulationoperations.

However, there are some concerns with regard to OFDM. Such concernsinclude high Peak-to-Average Power Ratio (“PAPR”) and frequency offset.A high PAPR causes saturation in power amplifiers, leading tointermodulation products among the subcarriers and disturbances ofout-of-band energy. Therefore, it is desirable to reduce the PAPR. Inorder to meet the out-of-band emissions requirements, a power amplifierand other components with this high PAPR input are required to providegood linearity in a large dynamic range. This power requirement makesthe power amplifier one of the most expensive components within thecommunication system. The high PAPR also means that the power amplifieroperation has low power efficiency that reduces battery life for relatedmobile stations. An elevated PAPR for infrastructure amplifiersincreases power consumption and heat generation, compromising systemreliability and limiting deployment options due to system coolingrequirements.

An OFDM signal exhibits a high PAPR because the independent phases ofthe sub-carriers mean that the sub-carrier signals may often combineconstructively allowing the peak of the signal to be up to N times theaverage power (where N is the number of sub-carriers). These large peaksincrease the amount of intermodulation distortion resulting in anincrease in the error rate. The average signal power must be kept low inorder to prevent transmitter amplifier gain limiting. Minimizing thePAPR allows a higher average power to be transmitted for a fixed peakpower, improving the overall signal to noise ratio at the receiver. Itis therefore desirable to reduce or otherwise minimize the PAPR.

Traditionally, in order to handle a high PAPR, a system uses a linearsignal chain. Any non-linearity in the signal chain will causeintermodulation distortion and degrades signal quality. The linearityrequirement is demanding, especially for transmitter RF output circuitrywhere amplifiers are often designed to be non-linear in order tominimize power consumption.

Prior PAPR reduction methods may be classified into two groups includingConstellation Shaping (“CS”), e.g., distortionless or activeconstellation expansion, and Tone Reservation (“TR”). With CS methods,the modulation constellation is changed such that the obtained PAPR isless than the required value with the satisfied channel error criteria.With TR methods, the reserved tones are assigned with such values thatthe obtained PAPR is less than the required value with the satisfiedchannel error criteria. In the tone reservation method, the idea is toreserve a small set of tones, or sub-carriers, for PAPR reduction. Theamount of PAPR reduction depends on the number of reserved tones, theirlocations within the frequency vector, and the amount of complexity.Other methods of reducing PAPR are also possible but they affect signalquality or Error-Vector Magnitude (“EVM”). One such method is disclosedin United States Patent Publication No. 2007/0140101, to Guo et al.,published Jun. 21, 2007 and entitled “System and Method for ReducingPeak-to-Average Power Ratio in Orthogonal Frequency DivisionMultiplexing Signals using Reserved Spectrum,” the entire teachings ofwhich are hereby incorporated by reference.

In practical OFDM systems, a small amount of peak clipping is allowed tolimit the PAPR in a tradeoff against linearity and power consumption.However, the transmitter output filter, which is required to reduceout-of-band spurs to legal levels, has the tendency to regrow peaklevels that were clipped, thus clipping alone has not been an effectiveway to reduce PAPR. One method of TR PAPR reduction clips the peaksignal and limits signal re-growth by distributing the excess clippedenergy among the known reserved sub-carriers while preventing thisdistortion or noise to affect any active sub-carriers, i.e.,sub-carriers carrying user data. However, at any given time, not every“active” or non-reserved sub-carrier is actually carrying data or wouldbe adversely affected by the addition of a small amount of noise.Additionally, although having a high number of reserved sub-carriers forenergy distribution aids in reducing the overall PAPR, the result isthat there are fewer sub-carriers available to carry data. Thus, priorTR methods also decrease the potential system capacity.

Therefore, what is needed is a system and method for reducing thepeak-to-average power ratio of OFDM communication systems by adaptivelydistributing excess clipped energy among reserved and activesub-carriers.

SUMMARY OF THE INVENTION

The present invention advantageously provides a method and system forreducing the peak-to-average power ratio in orthogonal frequencydivision multiplexing communication systems. Generally, the peak energyof a signal carrying an OFDM symbol is clipped and the clipped energy isdistributed among a combination of reserved sub-carriers anddata-carrying sub-carriers. The clipped signal is passed through afilter created in the frequency domain which has coefficients determinedbased on known information from a scheduler concerning the compositionof the signal, e.g., the total number of sub-carriers, the amount ofsub-carriers for each modulation scheme, whether a sub-carrier currentlycarries data, etc.

In accordance with one embodiment of the present invention, a method isprovided for adaptively reducing a peak-to-average power ratio in acommunication system. Energy is clipped from at least one peak of amodulated signal. The modulated signal includes a plurality ofsub-carriers. At least one data sub-carrier is adaptively selected forpeak-to-average power ratio reduction use based on known schedulinginformation. The clipped energy is adaptively distributed to at leastone data sub-carrier.

In accordance with another aspect of the present invention, a system foradaptively reducing a peak-to-average power ratio in a communicationsystem includes a scheduler, a clipper and a filter. The scheduleroperates to adaptively select at least one data sub-carrier for peak toaverage power ratio reduction. The clipper operates to clip energy fromat least one peak of a modulated signal. The modulated signal includesan orthogonal frequency division multiplexing symbol having a pluralityof sub-carriers. The filter is communicatively coupled to the schedulerand to the clipper. The filter operates to adaptively distribute theclipped energy among to at least one data sub-carrier.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention, and theattendant advantages and features thereof, will be more readilyunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram of an exemplary peak-to-average power ratioreduction system constructed in the frequency domain in accordance withthe principles of the present invention;

FIG. 2 is a graph of an exemplary F-filter response constructed inaccordance with the principles of the present invention;

FIG. 3 is a block diagram of an alternative peak-to-average power ratio(“PAPR”) reduction system constructed in the time domain in accordancewith the principles of the present invention; and

FIG. 4 is a graph of an exemplary g-function response constructed inaccordance with the principles of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Before describing in detail exemplary embodiments that are in accordancewith the present invention, it is noted that the embodiments resideprimarily in combinations of apparatus components and processing stepsrelated to implementing a system and method for reducing thepeak-to-average power ratio (“PAPR”) by adaptively distributing excessclipped energy among reserved and active sub-carriers in orthogonalfrequency-division multiplexing (“OFDM”) communication systems.Accordingly, the system and method components have been representedwhere appropriate by conventional symbols in the drawings, showing onlythose specific details that are pertinent to understanding theembodiments of the present invention so as not to obscure the disclosurewith details that will be readily apparent to those of ordinary skill inthe art having the benefit of the description herein.

As used herein, relational terms, such as “first” and “second,” “top”and “bottom,” and the like, may be used solely to distinguish one entityor element from another entity or element without necessarily requiringor implying any physical or logical relationship or order between suchentities or elements.

One embodiment of the present invention advantageously provides a methodand system for reducing the PAPR by distributing excess energy amongreserved and active sub-carriers in OFDM communication systems.Generally, a scheduler assigns users to different sub-carriers withdifferent modulation schemes and tracks the current assignments. Thisinformation is used to interactively determine those sub-carriers onwhich additional noise is acceptable, as well as the amount ofacceptable noise. The reduced PAPR reduces the overall system costs byallowing use of lower power amplifiers as well as increasing theflexibility of system configuration. Additionally, an embodiment of thepresent invention increases the overall system capacity by decreasingthe number of sub-carriers reserved solely for excess noise effects anddistributing these effects to active sub-carriers at a time when thenoise has minimal negative impact on the system performance.

Examples of OFDM communication systems include, but are not limited to,wireless protocols such as the wireless local area network (“WLAN”)protocol defined according to the Institute of Electrical andElectronics Engineering (“IEEE”) standards radio 802.11a, b, g, and n(hereinafter “Wi-Fi”), the Wireless MAN/Fixed broadband wireless access(“BWA”) standard defined according to IEEE 802.16 (hereinafter “WiMAX”),the mobile broadband 3GPP Long Term Evolution (“LTE”) protocol havingair interface High Speed OFDM Packet Access (“HSOPA”) or Evolved UMTSTerrestrial Radio Access (“E-UTRA”), the 3GPP2 Ultra Mobile Broadband(“UMB”) protocol, digital radio systems Digital Audio Broadcasting(“DAB”) protocol, Hybrid Digital (“HD”) Radio, the terrestrial digitalTV system Digital Video Broadcasting-Terrestrial (“DVB-T”), the cellularcommunication systems Flash-OFDM, etc. Wired protocols using OFDMtechniques include Asymmetric Digital Subscriber Line (“ADSL”) and VeryHigh Bitrate Digital Subscriber Line (“VDSL”) broadband access, Powerline communication (“PLC”) including Broadband over Power Lines (“BPL”),and Multimedia over Coax Alliance (“MoCA”) home networking.

The sub-carriers of the OFDM carriers may be modulated using a varietyof modulation schemes, such as Binary phase-shift keying (“BPSK”),Quadrature phase-shift keying (“QPSK”), Quadrature amplitude modulation(“16QAM” or “64QAM”), etc.

Referring now to the drawing figures in which like reference designatorsrefer to like elements, there is shown in FIG. 1, a peak-to-averagepower ratio reduction (“PAPRR”) system constructed in accordance withthe principles of the present invention and designated generally as“10.” PAPRR system 10 includes a scheduler 12 which determines when eachOFDM symbol of a data packet is to be transmitted and maps the data ontoan OFDM signal prior to modulation by the modulator 14. The scheduler 12also assigns users to different sub-carriers with different modulationschemes, tracks the current assignments, and interacts with aconfiguration unit 16, such as a controller or processor, to determinewhich sub-carriers may be available for PAPR reduction.

The system throughput is generally a function of the schedulerproperties and the number of reserved sub-carriers. In other words,Throughput=ƒ₁(N_(rsch), Scheduler). A small number of reservedsub-carriers generally increases the system throughput at the expense ofPAPR. Additionally, the scheduler 12 implements some type of fairnessmechanism, such as proportional or weighted fairness. With a fairscheduler, an OFDM symbol may consist of sub-carriers with higher order,e.g., 64QAM, as well as lower order modulations, e.g., QPSK.

The residue constellation error (“RCE”), also known as error vectormagnitude (“EVM”) for certain protocols, is a measurement that reflectsthe distance from each actual data point to its ideal constellationpoint. RCE is a function of the PAPRR algorithm used, the number ofreserved sub-carriers, the number of sub-carriers for differentmodulation schemes (e.g., 64QAM, 16QAM, etc.), and the schedulerproperties. In other words, RCE=ƒ₂(Algo_(PAPRR), N_(rech), N_(64QAM),N_(16QAM), Scheduler). The PAPRR algorithm controls the levels ofconstellation errors on different modulations according to therequirements. Generally, a larger number of reserved sub-carriers isneeded in order to have a smaller RCE.

The scheduler 12 limits the number of symbols with higher ordermodulation, e.g., 64QAM. The scheduler 12 determines the number ofreserved sub-carriers based, in part, on the number of symbols withhigher order modulations and RCE requirements.

It should be noted that, in accordance with one embodiment of thepresent invention, constellation errors and sub-carriers are defined inthe frequency domain. Scheduling and the bulk of actual processing arealso performed in the frequency domain. Additionally, OFDM symbols aremodulated onto an OFDM carrier signal in the frequency domain.

The modulated signal is converted to the time domain by passing thesignal through an inverse fast Fourier transform (“IFFT”) 18. A switch20 operates to capture an OFDM symbol for PAPRR processing prior totransmission. When switch 20 is in position A, a single OFDM symbol ispassed through from the modulator 14. The switch 20 then transitions toposition B during the PAPRR processing of the OFDM symbol. A secondswitch 21, when in the open position, prevents the OFDM symbol frombeing transmitted until after the PAPRR processing has been completed.

The OFDM signal, in the time domain, is passed through a clipper 22which clips the peaks of the signal, thereby limiting the peak power.Based on performance requirements received from the scheduler 12, theconfiguration unit 16 defines the target output PAPR level at the outputby setting the clipping threshold, TH_(clipping), used by the clipper22. The signal containing the energy clipped from the peaks istransformed back to the frequency domain by a fast Fourier transform(“FFT”) 24 and passed through an energy distributor filter 26 havingcoefficients determined by an F-filter generator 28 according toinformation from the scheduler 12 and the configuration unit 16regarding the number of sub-carriers and their associated modulationscheme or reserved sub-carrier status. The energy distributor filter 26distributes noise or distortion created by the clipping process ontosub-carriers where the scheduler 12 has indicated the additional noisemay be tolerated. The filtered signal is transformed back to the timedomain by a second IFFT 30 and combined with the original signal througha subtracter 32 which, in fact, removes the filtered clipped signal fromthe original modulated signal before being fed back through the clipper22 for additional iterations. Ideally, the number of iterationsapproaches infinity; however, practical applications limit the number ofiterations to generally about three or four. Further iterations do notyield significantly improved results, and are generally not needed.

The configuration unit 16 selects whether the number of iterations isfixed or variable, determines a re-growth control factor, σ, and definesthe maximum and minimum amount of resources available for PAPRR use,e.g., in WiMAX, the total number of sub-carriers. The re-growth controlfactor σ is used to control the re-growth of the peaks after clippingand optimize the performance of the PAPRR process. The re-growth controlfactor discussed in greater detail below. The configuration unit 16 alsodefines the resource reservation function, ƒ_(RR)( ), which determinesthe amount, N_(rsch), of sub-channels reserved for PAPRR purposesaccording to the amount of sub-carriers modulated with specificmodulation schemes. In other words, N_(rsch)=ƒ_(RR)(N_(64QAM),N_(16QAM), N_(QPSK)). The configuration unit 16 communicates theresource reservation function to the scheduler 12 which uses thisfunction in scheduling user data onto sub-carriers.

For example, in the hypothetical case of a WiMAX communication systemwhich uses 64QAM modulation exclusively, the number of sub-channelsreserved for PAPR may be simply a function of total number ofsub-channels having 64QAM modulated sub-carriers. Thus,

$\begin{matrix}{N_{rsch} = {{f_{RR}\left( N_{64\;{QAM}} \right)} = \left\{ \begin{matrix}{0,{{N_{64\;{QAM}} \leq 2};}} \\{1,{{2 < N_{64\;{QAM}} \leq 4};}} \\{2,{{4 < N_{64\;{QAM}} \leq 6};}} \\{3,{6 < {N_{64\;{QAM}}.}}}\end{matrix} \right.}} & (1)\end{matrix}$

Therefore, if the system has less than two N_(64QAM) sub-channels with64QAM modulation, no reserved sub-channels are required. If the systemhas between 2 and 4 N_(64QAM) sub-channels with 64QAM modulation, onereserved sub-channel is needed, and so on.

Table 1 illustrates a more complex example of a WiMAX communicationsystem which uses several modulation schemes. In this example, thenumber of reserved sub-carriers may be stored in the form of a look-uptable.

TABLE 1 Number of Reserved Sub-channels Required for DifferentModulation Schemes N_(rsch) N_(QPSK) N_(16QAM) N_(64QAM) 0 1 1 1 1 4 6 5. . . . . . . . . . . . N_(rsch) N_(QPSK) N_(16QAM) N_(64QAM)

Thus, according to the example shown in Table 1, with one reservedsub-channel, the system may support up to 4 QPSK sub-channels, 6 16QAMsub-channels, and 5 64QAM sub-channels and still meet its desired PAPRgoals.

Additionally, the configuration unit 16 defines an energy distributionfunction, ƒ_(ED)( ), based on the RCE or signal quality requirements.The energy distribution function determines a vector, F, of the weightson the pilot, the reserved sub-carriers, and the data sub-carriersmodulated with different orders as F=ƒ_(ED)(RCE_(64QAM), RCE_(16QAM),RCE_(QPSK)).

The scheduler 12 maintains a record of the quantity of sub-carriersassigned to the different modulation orders during scheduling, as wellas the specific sub-carriers that have been assigned to the differentmodulation orders. Based on the distribution or combination of theresource amounts assigned to the different modulation orders, thescheduler 12 decides whether a unit of a resource, e.g., a particularsub-channel, shall be reserved for PAPRR purposes. The scheduler 12communicates the mapping of modulated and PAPR reserved sub-carriers tothe F-filter generator 28.

The F-filter generator 28 obtains the energy distribution functionƒ_(ED)( ) and the re-growth control factor σ from the configuration unit16, and the sub-carrier mapping for each OFDM symbol from the scheduler12. The F-filter generator 28 uses the energy distribution functionƒ_(ED)( ), the re-growth control factor σ, and the sub-carrier mappingto generate the coefficients to construct an energy distributor filter26 for each OFDM symbol.

One embodiment of the present invention calculates the F-filtercoefficients by assuming that L modulation schemes exist within an OFDMsymbol having k₁, k₂, . . . , k_(L) sub-carriers for each modulationscheme, respectively, then

$\begin{matrix}{N_{sc} = {\sum\limits_{i = 1}^{L}\;{k_{i}.}}} & (2)\end{matrix}$The average energy of sub-carriers of scheme i, where i=1, 2, . . . , L,is denoted as E_(i). A power boost ratio of the sub-carriers with i-thscheme against 1^(st) scheme requires dE_(j), j=1, 2, . . . , L, where

$\begin{matrix}{{dE}_{j} = {\frac{E_{j}}{E_{1}}.}} & (3)\end{matrix}$The set of sub-carriers with i-th modulation scheme is denoted asSCS_(i). K schemes are used for PAPR reduction, and, without loss ofgenerality, modulation scheme 1 has the highest order. Each scheme hask₁, k₂, . . . , k_(K) sub-carriers, respectively. The ResidualConstellation Error requirements of each modulation scheme is denoted asRCE₁, RCE₂, . . . , RCE_(K), respectively, and are defined as

$\begin{matrix}{{{RCE}_{i} = \frac{{Eclip}_{i}}{E_{i}}},} & (4)\end{matrix}$where Eclip_(i) denotes the average clipping interference energy on asub-carriers of scheme i, where i=1, 2, . . . , K. The RCE ratios aredenoted as dRCE_(j), where j=2, . . . , K, and may be determined by

$\begin{matrix}{{dRCE}_{j} = {\frac{{RCE}_{j}}{{RCE}_{1}}.}} & (5)\end{matrix}$

The following calculations are performed:Eclip_(j) =Eclip_(i) ·dE _(j) ·dRCE_(j);  (6)Aclip_(j)=(Eclip_(j))^(1/2);  (7)

-   -   with Eclip₁=1.

The F-filter coefficients F={F_(i)|i=0, . . . N−1} are determined byfinding F_(i), where

$\begin{matrix}{F_{i} = \left\{ {\begin{matrix}{{Aclip}_{1},{i \in {SCS}_{1}}} \\{{Aclip}_{2},{i \in {SCS}_{2}}} \\\vdots \\{{Aclip}_{K},{i \in {SCS}_{K}}} \\{0,\mspace{14mu}{otherwise}}\end{matrix},} \right.} & (8)\end{matrix}$and normalizing the result.

A re-growth control factor σ is used to reach the lowest PAPR with therequired signal quality using the fewest number of iterations. When theclipped signal is converted to the frequency domain by the FFT 24,passed through the energy distributor filter 26, IFFT 30 and subtracter32, the cancellation is imperfect, which has a tendency to push up thepeak amplitude, in effect re-growing the peak. The re-growth controlfactor may be viewed as an “aggressiveness factor” which determines howquickly, i.e., how many iterations are needed, to reach the PAPR goal.The value of the re-growth control factor σ is between 0 and 1 and isused to scale the coefficients. By using an appropriate σ, lessiterations are needed to reach the desired PAPR. The re-growth controlfactor σ is determined based on the number of reserved sub-carriersN_(rsch), the combination of sub-carriers in different modulation orders(in numbers as well as positions), the iteration order i_(ite), and thesignal quality requirements. In other words, σ(i_(ite))=ƒ_(σ)(N_(rsch),i_(ite), N_(64QAM), N_(16QAM), N_(QPSK)). Therefore, the re-growthfactor can vary to adapt to the signal transmitted.

An exemplary energy distributor filter 26 is described with reference toFIG. 2. The energy distributor filter 26 is a frequency domain filterwith F-filter coefficients. The energy distributor filter 26 selectivelyspreads the clipped energy from the peaks of the OFDM symbol ontosub-carriers according to the energy distribution function in order tosatisfy the RCE requirements. Referring to the filter shown in FIG. 2,it should be noted that there are basically four levels of coefficientsfor this exemplary filter. The highest level 34 has a coefficient ofapproximately 19, thereby distributing the majority of the clippedenergy to those sub-carriers designated with this coefficient. A secondlevel 36 has a coefficient of approximately 10, and a third level 38 hasa coefficient of approximately 1.5. The excess energy clipped from thepeak OFDM signal is thus distributed across the carrier spectrum inproportion to the coefficient weights. It should be noted thatsub-carriers near the band edges receive a coefficient of 0, i.e., level40, thereby preventing any excess noise from being distributed to thosesub-carriers

Referring now to FIG. 3, an alternate embodiment of the presentinvention is provided wherein the PAPRR system 42 performs the actualfiltering of the clipped OFDM signal in the time domain. As with theprevious embodiment, a scheduler 12 determines when each OFDM symbol ofa data packet is to be transmitted and maps the data onto an OFDM signalprior to modulation by the modulator 14. The scheduler 12 interacts witha configuration unit 16 to determine which sub-carriers are availablefor PAPR reduction. The modulated signal is converted to the time domainby passing the signal through an inverse fast Fourier transform (“IFFT”)18. A switch 20 operates to capture an OFDM symbol for PAPRR processingprior to transmission. When switch 20 is in position A, a single OFDMsymbol is passed through from the modulator 14. The switch 20transitions to position B during the PAPRR processing of the OFDMsymbol. The OFDM signal, in the time domain, is passed through a clipper22 which clips the peaks of the signal, thereby limiting the peak power.A second switch 21, when in the open position, prevents the OFDM signalfrom being transmitted until all iterations of the PAPRR processing arecompleted.

Based on performance requirements received from the scheduler 12, theconfiguration unit 16 defines the target output PAPR level at the outputby setting the clipping threshold, TH_(clipping), used by the clipper22. In this embodiment, energy distributor filter 26 still has itscoefficients determined in the frequency domain by an F-filter generator28 based on information from the scheduler 12 and the configuration unit16 regarding the number of sub-carriers and their associated modulationschemes; however, the energy distributor filter 26 is transformed by anIFFT 44 into a g-function filter 46 which applies the filtering to theclipped signal in the time domain. As in the example above, the filteredsignal is combined with the original signal through a subtracter 32 andthe process is repeated for a number of iterations determined by theconfiguration unit 16. FIG. 4 illustrates the exemplary F-filter of FIG.2, transformed to a complimentary g-function filter.

The adaptive PAPRR process of the present invention advantageouslyprovides higher throughput than prior PAPRR methods, thus increasingsystem capacity. The present invention also provides better performancein PAPR reduction compared with prior art methods and is able to adaptto changes in the system loading while maintaining tight control on thesignal quality, e.g., RCE. On-line generation of F-filter coefficientsallows for a flexible configuration without greatly increasing thecomplexity of the PAPRR process.

The present invention can be realized in hardware, software, or acombination of hardware and software. Any kind of computing system, orother apparatus adapted for carrying out the methods described herein,is suited to perform the functions described herein.

A typical combination of hardware and software could be a specialized orgeneral purpose computer system having one or more processing elementsand a computer program stored on a storage medium that, when loaded andexecuted, controls the computer system such that it carries out themethods described herein. The present invention can also be embedded ina computer program product, which comprises all the features enablingthe implementation of the methods described herein, and which, whenloaded in a computing system is able to carry out these methods. Storagemedium refers to any volatile or non-volatile storage device.

Computer program or application in the present context means anyexpression, in any language, code or notation, of a set of instructionsintended to cause a system having an information processing capabilityto perform a particular function either directly or after either or bothof the following a) conversion to another language, code or notation; b)reproduction in a different material form.

In addition, unless mention was made above to the contrary, it should benoted that all of the accompanying drawings are not to scale.Significantly, this invention can be embodied in other specific formswithout departing from the spirit or essential attributes thereof, andaccordingly, reference should be had to the following claims, ratherthan to the foregoing specification, as indicating the scope of theinvention.

What is claimed is:
 1. A method for adaptively reducing apeak-to-average power ratio in a mobile station, the method comprising:receiving a modulated signal, the modulated signal comprising aplurality of data sub-carriers; clipping the modulated signal, whereinsaid clipping includes clipping energy from at least one peak of themodulated signal; distributing the clipped energy among at least one ofthe data sub-carriers, wherein the clipped energy is distributed using are-growth control factor, wherein a value of the re-growth controlfactor is between 0 and 1, wherein the re-growth control factor is usedto control re-growth of the at least one peak after clipping; anditeratively repeating the distribution based on the regrowth controlfactor.
 2. The method of claim 1, wherein the modulated signal includesan orthogonal frequency division multiplexing symbol, wherein the methodfurther comprises transmitting the modulated signal after saiditeratively repeating.
 3. The method of claim 1, wherein the datasub-carriers are modulated using a plurality of modulation schemes. 4.The method of claim 1, further comprising distributing the clippedenergy among at least one reserved sub-carrier.
 5. The method of claim1, wherein the modulated signal includes an orthogonal frequencydivision multiplexing symbol and wherein the method further comprises:assigning communication devices to the data sub-carriers; mapping theorthogonal frequency division multiplexing symbol onto the modulatedsignal; and determining the at least one data sub-carrier fordistributing the clipped energy based on said assigning thecommunication devices.
 6. The method of claim 5, further comprising:transforming the clipped signal for use in a frequency domain; filteringthe transformed signal using the filter; transforming the filteredsignal for use in a time domain; and combining the transformed filteredsignal with the modulated signal to produce an output signal having apeak-to-average power ratio less than the peak-to-average power ratio ofthe modulated signal.
 7. The method of claim 6, wherein the filterincludes a weighted coefficient for each sub-carrier.
 8. The method ofclaim 7, wherein the coefficient for each sub-carrier is determinedbased at least in part on said assigning the communication devices, atotal amount of sub-carriers for the modulated signal, an amount ofsub-carriers for each modulation scheme, an amount of reservedsub-carriers and signal quality parameters.
 9. The method of claim 8,wherein the signal quality parameters include the peak-to-average powerratio for the output signal and residual constellation error for eachsub-carrier.
 10. The method of claim 5, further comprising: filteringthe clipped modulated signal using a g-function; and combining thefiltered clipped energy with the clipped modulated signal to produce anoutput signal having a peak-to-average power ratio less than thepeak-to-average power ratio of the modulated signal.
 11. The method ofclaim 10, wherein the g-function is constructed by: constructing anF-filter in a frequency domain, the F-filter comprising a weightedcoefficient for each sub-carrier; and transforming the F-filter to ag-function for use in a time domain.
 12. The method of claim 11, whereinthe coefficient for each sub-carrier is determined at least in part onsaid assigning the communication devices, a total amount of sub-carriersfor the modulated signal, an amount of sub-carriers for each modulationscheme, an amount of reserved sub-carriers and signal qualityparameters.
 13. The method of claim 1, wherein the re-growth controlfactor is determined based at least in part on a number of reservedsub-carriers.
 14. A system for adaptively reducing a peak-to-averagepower ratio in a mobile station, the system comprising: a clipperconfigured to clip a modulated signal, wherein said clipping includesclipping energy from at least one peak of the modulated signal, themodulated signal comprising a plurality of data sub-carriers; and afilter, communicatively coupled to the clipper, wherein the filter isconfigured to: adaptively distribute the clipped energy among the atleast one of the data subcarriers, the filter having coefficients basedat least in part on a re-growth control factor, wherein the re-growthcontrol factor is used to scale the coefficients of the filter, whereina value of the re-growth control factor is between 0 and 1, wherein there-growth control factor is used to control re-growth of said at leastone peak after said clipping; and iteratively repeat the distributionbased on the regrowth control factor.
 15. The system of claim 14,wherein the filter is further configured to adaptively distribute theclipped energy among at least one reserved sub-carrier.
 16. The systemof claim 14, further comprising: a scheduler configured to adaptivelyselect said at least one data sub-carrier for peak to average powerratio reduction; and a transmitter configured to transmit the modulatedsignal after said iteratively repeating.
 17. The system of claim 14,wherein the modulated signal includes an orthogonal frequency divisionmultiplexing symbol, wherein the system further comprises a schedulerand a filter generator communicatively coupled to the scheduler, whereinthe filter generator is configured to create the filter, and wherein thescheduler is configured to: adaptively select said at least one datasub-carrier for peak to average power ratio reduction; assigncommunication devices to said at least one data sub-carrier; map theorthogonal frequency division multiplexing symbol onto the modulatedsignal; wherein said selecting the at least one data sub-carrier isbased on said assigning the communication devices.
 18. The system ofclaim 17, wherein the filter is an F-filter, the F-filter including aweighted coefficient for each sub-carrier, the system furthercomprising: a fast Fourier transform unit communicatively coupled to theclipper and to the filter, the fast Fourier transform unit configured totransform the clipped modulated signal for use in a frequency domain; aninverse fast Fourier transform unit communicatively coupled to thefilter, wherein the inverse fast Fourier transform unit is configured totransform the filtered clipped signal for use in a time domain; and asubtracter, communicatively coupled to the inverse fast Fouriertransform unit and to the clipper, wherein the subtracter is configuredto combine the transformed filtered clipped signal with the modulatedsignal to produce an output signal having a peak-to-average power ratioless than the peak-to-average power ratio of the modulated signal. 19.The system of claim 18, wherein the filter generator is configured todetermine the coefficient for at least one sub-carrier based at least inpart on said assigning the communication devices, a total amount ofsub-carriers for the modulated signal, an amount of sub-carriers foreach modulation scheme, an amount of reserved sub-carriers, the peak-toaverage power ratio for the output signal, and residual constellationerror for at least one sub-carrier.
 20. The system of claim 19, furthercomprising a configuration unit communicatively coupled to the schedulerand to the filter generator, wherein the configuration unit isconfigured to: determine a number of iterations and a clipping thresholdfor the clipper; determine the re-growth control factor; and define amaximum and a minimum amount of sub-carriers available forpeak-to-average power ratio reduction.